Stub out Document::syncAnnotScan method.
The PDF specification  says:
syncAnnotScan guarantees that all annotations will be scanned
by the time this method returns.
Normally a background task runs that examine every page and
looks for annotations during idle times.
The statement details specifically how Acrobat implements
Although, neither the method itself nor the background scanner
task are implemented in PDFium (as of today, Ago/2016),
not having ::syncAnnotScan at least stubbed out can be considered
harmfull since its absence makes JS acrobat scripts silently
fail when it has a call to it.
Given that, and following a stub-out pattern present in other
methods including ::addAnnot and ::addField, CL provides
a stubbed out implementation of Document::syncAnnotScan.